Sealants are used in many industry sectors including building and construction, mining and transport, e.g., in aircraft, automotive vehicles and marine craft. The physical and chemical properties of a sealant may be tuned through design of the sealant polymer chemistry and through the use of additives in the formulation to meet any desired performance requirements, e.g., to obtain suitable rheological properties for deployment, cure kinetics, bond strength to a substrate, mechanical properties and chemical resistance.
Among their use in a range of applications, sealants are particularly relevant for the construction of modern aircraft, for example, for sealing fasteners, for obtaining fluid-tight seals in fuel tanks and for pressure seals in joints or surrounding fasteners. Chemical resistance is a fundamental requirement of aircraft sealants since they may be exposed to chemicals such as jet fuel, hydraulic fluids and cleaning agents in service. Sealants that incorporate sulfur into a polymer backbone, such as in polysulfide and polythioether based sealant formulations, are known to provide suitable performance characteristics for many aerospace applications.
When polysulfides or polythioethers are mixed with a curing agent, sealant work life (i.e., the time during which the material can be applied) begins. Sealant work life ends when the properties of the sealant (such as its rheological properties) are no longer suitable for the application method. Variation in cure rate may increase or decrease work life and the time required to reach a tack free state (i.e., sealant is no longer sticky to touch) and ultimate hardness.
In terms of deployment of the sealant in a manufacturing environment, such as in the manufacture of an aircraft, this cure phenomena inevitably leads to application and scheduling complexity and hence implications for work flow and productivity. Having a simple and reliable indicator, which could provide a distinct visual signal, signifying the progress and/or the end of a cure, may shorten the time between application of sealant and the beginning of a re-work cycle as needed. This can reduce some work to a single shift, or allow secondary sealant work to be performed at earlier points on the production line when access (for the work and the inspection) is easier. However, the present indicator molecules, which degrade and change color from blue to white, for example, upon exposure to heat and/or UV are often difficult to observe due to the low contrast between the two colors. Additionally, the color change does not necessarily directly correlate to the extent of cure. Accordingly, there remains a desire in the art for an indicator molecule having a readily observable color change, which corresponds to the curing progress of a sealant composition.